we use 2g NSN network too , and SDCCH drop rate for a few sector extremely high.
NSN support team suggested to :
1- Check whether Abis interface is stable using PA.
2- Add another SDCCH channel to the TRX suffering from SDCCH drop
3- Change TS of SDDCH to another TS in TRX
4- Replace BB2 & TRX with new ones
5- Change connector & Cable and even antenna if SDCCH drop continues.
6- Measure interferance,
7- ensure that there is no POWER issue in the site, ( DC power fluctuation)
8- Poor Coverage issue, may be the reason…. etc
i have observed that SDDCH drop as you mentioned it is related to Abis failure
Main reason in Abis Fialure is T3101 timer expiry,
i don’t know how to improve T3101 timer expiry situation ,
if you have any idea , pls share it with us ,